This invention relates to semiconductor devices in general, and particularly to bipolar semiconductor devices of the type used for controlling the flow of electrical power.
In the design of bipolar semiconductor power devices, there is generally a trade-off between low on-state voltage drop across the device and fast switching time, particularly fast turn-off of the device. Low voltage drop is achieved by bipolar operation, i.e., the introduction of both p and n type charge carriers through the device for obtaining large charge carrier densities. Large densities cause the device to operate in a "conductivity modulated" mode providing a low resistance to current flow. Conversely, because switching off the device requires removal of charge carriers from within the device, large current densities tend to increase device turn-off time.
The trade-off generally accepted is the use of bipolar operation for high charge carrier density, but operation at a level of charge density somewhat reduced from the maximum density otherwise available. Typically, for example, two "areas" of origination of charge carriers are present in each device, a first of which is under the "direct" control of a nearby control electrode, and the second of which is spaced from the first "area" and which operates in response to the turn-on and turn-off of the first area. (As described hereinafter, the term "area of origination" is intended to cover both "source regions" in field-effect devices and "emitter regions" in high-low and p-n junction devices.)
In all devices in which the present invention has utility, the second "area" is a p-n junction across which charges are injected by an "emitter". Typical techniques for reducing the amount of charge injected across a p-n junction include reducing the effective size of the junction, e.g., by providing charge carrier paths ("shorts") bypassing the p-n junction, and/or selective doping of the emitter region and/or the adjoining region forming the p-n junction with the emitter region. The shorted emitter construction tends to be complex and relatively expensive and the selective doping, depending upon the doping scheme used, can also add complexity and cost to the device or be otherwise undesirable in terms of device performance or manufacture.
Another technique for improving device turn-off speed is to reduce the life-time of the charge carriers within the device, e.g., by the introduction of carrier recombination sites. However, this is undesirable for, among other things, adding extra processing and cost to the devices.
The present invention is directed to a different mechanism for obtaining reduced injection efficiency of the second "area of origination" of charge carriers within the devices, that is, the p-n junction emitter region remote from the gate controlled first "area" of the devices, which different mechanism provides certain advantages in device performance and device manufacture. The "different mechanism" hereinafter described can, depending upon the particular semiconductor device being made, be used in combination with known mechanisms, e.g., lifetime control mechanisms.